This invention relates to the polymerization of a mono-1-olefin, such as ethylene, using a catalyst system comprising a supported chromium oxide catalyst.
Supported chromium catalysts have long been a dominate factor in the production of high density olefin polymers such as polyethylene. As originally commercialized, these catalysts were used in solution polymerization processes. However, it really became evident that a more economical route to many commercial grades of olefin polymers was a slurry process, that is, a polymerization process carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent. However, certain control techniques which are easily carried out in solution polymerization systems became more difficult in the slurry, or particle form, system. This is particularly true with regard to control of the molecular weight of the polymer. In a solution system, the temperature can be increased in order to provide a lower molecular weight and a higher melt flow polymer. However, in slurry systems, there is a practical limit on temperature increases, since the point is quickly reached wherein the polymer goes into solution and thus the value of the slurry system is lost.
It is also known that mono-1-olefins, such as ethylene, can be polymerized with catalyst systems employing vanadium, chromium, or other metals on a support such as alumina, silica, aluminum phosphate, titania, ziconia, magnesia, or other refractory metal oxides. Initially such catalysts were used primarily to form homopolymers of ethylene. It soon developed, however, that many applications required polymers which were more impact resistant than ethylene homopolymers. Consequently, in order to produce a polymer having short chain branching like the more flexible, free radical polymerized ethylene polymers, comonomers such as propylene, butene, hexene or other higher olefins were copolymerized with the ethylene to provide resins tailored to specific end uses. Copolymers, however, are more expensive to produce since inventories of different monomers must be kept and also the comonomers are generally more expensive than ethylene. Linear ethylene polymers with short chain branching can be formed from a pure ethylene feed using the old free radical high pressure process, but the conditions necessary to do this make the product too expensive to be commercially competitive.
Additional control over the polymerization process and the resultant polymer is also desired. A process to consistently reduce the density of linear ethylene polymers and to more efficiently produce and incorporate comonomers into the linear ethylene polymer is economically advantageous.